[Java 8 HashMap 详解系列] 文章目录
1.HashMap 的存储数据结构
2.HashMap 中 Key 的 index 是怎样计算的?
3.HashMap 的 put() 方法执行原理
4.HashMap 的 get() 方法执行原理
5.HashMap 的 remove() 方法执行原理
6.HashMap 的扩容 resize() 原理
7.HashMap 中的红黑树原理
3.HashMap 的 put() 方法执行原理
HashMap 重要成员变量:
//初始值，为16，必须为2的次幂
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;
//当容量被占满0.75时就需要reSize扩容
static final float DEFAULT_LOAD_FACTOR = 0.75f;
//链表长度到8，就转为红黑树
static final int TREEIFY_THRESHOLD = 8;
// 树大小为6，就转回链表
static final int UNTREEIFY_THRESHOLD = 6;
HashMap类中有一个非常重要的字段，就是Node[] table，即哈希桶数组:
/**
* The table, initialized on first use, and resized as
* necessary. When allocated, length is always a power of two.
* (We also tolerate length zero in some operations to allow
* bootstrapping mechanics that are currently not needed.)
*/
transient Node<K,V>[] table;
Node是HashMap的一个内部类，实现了Map.Entry接口，本质就是一个映射(键值对) :
/**
* Basic hash bin node, used for most entries. (See below for
* TreeNode subclass, and in LinkedHashMap for its Entry subclass.)
*/
static class Node<K,V> implements Map.Entry<K,V> {
final int hash;
final K key;
V value;
Node<K,V> next;
Node(int hash, K key, V value, Node<K,V> next) {
this.hash = hash;
this.key = key;
this.value = value;
this.next = next;
}
public final K getKey() { return key; }
public final V getValue() { return value; }
public final String toString() { return key + "=" + value; }
public final int hashCode() {
return Objects.hashCode(key) ^ Objects.hashCode(value);
}
public final V setValue(V newValue) {
V oldValue = value;
value = newValue;
return oldValue;
}
public final boolean equals(Object o) {
if (o == this)
return true;
if (o instanceof Map.Entry) {
Map.Entry<?,?> e = (Map.Entry<?,?>)o;
if (Objects.equals(key, e.getKey()) &&
Objects.equals(value, e.getValue()))
return true;
}
return false;
}
}
/**
* Associates the specified value with the specified key in this map.
* If the map previously contained a mapping for the key, the old
* value is replaced.
*
* @param key key with which the specified value is to be associated
* @param value value to be associated with the specified key
* @return the previous value associated with <tt>key</tt>, or
* <tt>null</tt> if there was no mapping for <tt>key</tt>.
* (A <tt>null</tt> return can also indicate that the map
* previously associated <tt>null</tt> with <tt>key</tt>.)
*/
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
/**
* Implements Map.put and related methods
*
* @param hash hash for key
* @param key the key
* @param value the value to put
* @param onlyIfAbsent if true, don't change existing value
* @param evict if false, the table is in creation mode.
* @return previous value, or null if none
*/
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
Node<K,V>[] tab; Node<K,V> p; int n, i;
//1.初始化
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
//2.当前位置没有存储节点,则直接插入节点
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
Node<K,V> e; K k;
//3.hash 相等 && key 相等
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
//4.节点插入红黑树
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
//5.遍历链表
for (int binCount = 0; ; ++binCount) {
//如果到了链表尾部:(e = p.next) == null
if ((e = p.next) == null) {
//节点插入链表尾部
p.next = newNode(hash, key, value, null);
//如果节点个数>=TREEIFY_THRESHOLD,链表结构转为红黑树结构
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
// 找到了对应元素，就可以停止了, break
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
//继续遍历链表的下一个节点
p = e;
}
}
if (e != null) { // existing mapping for key
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
链表转成红黑树的操作 treeifyBin(tab, hash) 我们将在
7.HashMap 中的红黑树原理
详细讲解.
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